DK59935. Intracellular lipids, particularly triacylglycerol, fatty acids, aryl-CoAs and their metabolites, provide a critical link between obesity, insulin resistance and diabetes. Between 1990 and 1998 five different rat isoforms of acyl-CoA synthetase (ACS) were cloned and it has become apparent that the different ACS isoforms probably play major roles in regulating cellular fatty acid and acyl-CoA levels. Thus, it is surprising that little information is available about the individual ACS isoforms. Using non-cross-reacting peptide antibodies, we have shown that the three isoforms expressed in liver and adipocytes, ACS1, 4, and 5 are each located in different subcellular membranes in liver, that they are inhibited by different chemical inhibitors, and that they are regulated independently in liver by fasting and refeeding. Further, we discovered that thiazolidinediones specifically inhibit AVCS4, suggesting that these clinically important insulin sensitizers might act, in part, by inhibiting ACS4. We now propose to focus on the function, regulation, and structure of ACS1, 4, and 5 I order to understand how each of these ACSs contributes to normal glycerolipid metabolism an what role each ACS isoform plays in promoting the lipid-related pathophysiology of insulin resistance and diabetes. In order to determine the function of the ACSs , we will over express each of the three ACS isoforms, and use selective chemical and antisense inhibitors to assess effects on synthetic and degradative pathways. We will compare the cellular locations of the ACSs with confocal microscopy and determine whether regulation of ACS activities includes phosphorylation/dephosphorylation and movement from cytosol to intracellular membranes. We will determine the topography of the acyl-CoA synthetase isoforms within membranes and their crystal structures. Finally, we will use the yeast two-hybrid system to determine whether each ACS has one or more specific metabolic partners. These studies will enable us to understand how acyl- CoAs can serve as both metabolic signals and as substrates for synthetic and energy-producing pathways, how they can be partitioned towards different metabolic fates, and how their metabolism contributes to the pathogenesis of diabetes.